The present invention relates to a surface shape recognition sensor and a method of fabricating the same and, more particularly, to a surface shape recognition sensor for detecting fine three-dimensional patterns such as a human fingerprint and an animal noseprint and a method of fabricating the same.
Attention is now focusing on security techniques in the progression of the information-oriented society and environments in the present society. For example, in the information-oriented society, authentication techniques for the construction of an electronic cash system and the like have grown in importance. Recently, research and development have extensively made on authentication techniques for protection against theft and fraudulent use of cards (e.g., Yoshimasa Shimizu et al., "Study on IC Card Having Personal Authentication Function", Technical Report of IEICE, OFS92-32, pp. 25-30 (1992)).
Various authentication schemes for the prevention of fraud use fingerprints, voiceprints, and the like. Various fingerprint authentication techniques have been developed. These fingerprint authentication schemes are roughly classified into optical read schemes and schemes of detecting ridges/valleys on the skin of a finger by converting them into an electrical signal using human electrical characteristics.
An optical read scheme is a scheme of receiving a fingerprint as optical image data by mainly using reflection of light and a CCD image sensor and collating the fingerprint (Japanese Patent Laid-Open No. 61-221883). Another scheme that uses thin piezoelectric film to read the pressure difference between the ridge and valley on the skin of a finger has been developed (Japanese Patent Laid-Open No. 5-61965). As a scheme of detecting a fingerprint pattern by converting changes in electrical characteristics upon contact of the skin of a finger into an electrical signal distribution, an authentication scheme that uses pressure-sensitive sheets and is based on the change amounts of resistance and capacitance has been proposed (Japanese Patent Laid-Open No. 7-168930).
Of the techniques described above, the scheme using light is difficult to realize a compact system and hence is difficult to apply it for the general purpose. That is, the application of this scheme is limited. The scheme of detecting the ridges/valleys on the skin of a finger by using a pressure-sensitive sheet is difficult in practical use and has poor reliability because a special material is required and is difficult to process.
A capacitive fingerprint sensor based on LSI fabricating techniques has been developed (Marco Tartagni and Roberto Guerrieri, A 390 dpi Live Fingerprint Imager Based on Feedback Capacitive Sensing Scheme, 1997 IEEE International Solid-State Circuits Conference, pp. 200-201 (1997)). This is a method of detecting a ridge/valley pattern on the skin of a finger by the feedback capacitive sensing scheme using small sensors two-dimensionally arranged on an LSI chip. This capacitive sensor is designed such that two plates are formed on the uppermost LSI interconnection, and a passivation film is formed on the plates. When the tip of a finger touches this sensor, the surface of the skin serves as a third plate, and is isolated from the sensor through an insulating layer made of air. A fingerprint is detected by performing sensing operation in accordance with differences in distance between the ridges/valleys on the skin and the sensor. This structure allows a reduction in size because no special interface is required, as compared with the conventional optical scheme.
This fingerprint sensor is basically fabricated by forming sensor electrodes on a semiconductor substrate, and forming a passivation film on the sensor electrodes. In a method using this sensor, the capacitance between the skin and the sensor is detected through the passivation film to detect a fine ridge/valley pattern.
A conventional capacitive fingerprint sensor will be briefly described below with reference to FIGS. 24 and 25. This capacitive sensor has an arrangement like the one shown in FIG. 24. First of all, an interconnection 2403 is formed on a semiconductor substrate 2401, on which LSIs and the like are formed, through a lower insulating film 2402. An interlevel dielectric film 2404 is then formed on the interconnection 2403.
For example, sensor electrodes 2406 each having a rectangular planar shape are formed on the interlevel dielectric film 2404. Each sensor electrode 2406 is connected to the interconnection 2403 through a plug 2405 in a through hole formed in the interlevel dielectric film 2404. A passivation film 2407 is formed on the interlevel dielectric film 2404 to cover the sensor electrodes 2406, thereby forming sensor elements. These sensor elements are two-dimensionally arranged such that the sensor electrodes 2406 of the adjacent sensor elements do not come into contact with each other.
The operation of this capacitive sensor will be described. When a fingerprint is to be detected, a finger as a fingerprint detection target object touches the passivation film 2407. When the finger touches the film in this manner, the skin in contact with the passivation film 2407 serves as an electrode on the sensor electrodes 2406. As a result, capacitances are formed between the skin and the sensor electrodes 2406. The capacitances are detected through the interconnections 2403. A fingerprint is formed by the ridges/valleys on the skin of the finger. When, therefore, the finger touches the passivation film 2407, the distances from the skin serving as an electrode and the sensor electrodes 2406 differ in accordance with the ridges/valleys that form a fingerprint. These differences in distance are detected as differences in capacitance. By detecting a distribution of difference capacitances, the ridge pattern on the skin of the finger can be obtained. That is, a fine ridge/valley pattern on the skin can be detected by this capacitive sensor.
Such a capacitive fingerprint sensor can be made compact because no special interface is required, as compared with a conventional optical sensor.
This capacitive sensor can be mounted together with an LSI on the following integrated circuit (LSI) chip. More specifically, the above capacitive sensor can be mounted on an integrated circuit chip together with a storage section storing fingerprint data for collation and a recognition processing section integrated with the storage section to compare/collate fingerprint data prepared in a storage section with a read fingerprint. The formation of such components on one integrated circuit chip makes it difficult to tamper with information in data transfer between units, thus improving the security protection performance.
According to the above sensor, however, since the skin is used as an electrode, an LSI mounted on the substrate together with the sensor is susceptible to damage due to static electricity generated when the skin touches the sensor.
Demands have therefore arisen for a sensor for sensing a fine three-dimensional pattern such as a human fingerprint or animal noseprint and a method of fabricating the same, with consideration given to the stability, sensitivity, reliability, and the like of the sensor, miniaturization, and versatility.